1. Field of the Invention
The present invention relates to an image forming technique. Specifically, the present invention relates to an image forming processing technique that keeps continuity of unsupplied pages and supplied pages, even if the image forming processing is temporally suspended.
2. Description of the Related Art
As a field relating to a digital copying machine or printing, a field called “on demand print” has been drawing attention. On demand print can meet demands for various types of products by small lot and can also make the print contents to be changed easily. That is suitable for producing a document such as a manual or the like or a pamphlet.
Further, substantial reduction of print inventory or substantial shortening of man-hour enabled by inline completion from data input to book binding can be realized. It is also characterized in that substantial shortening of delivery time or reduction of a delivery cost can be realized based on simplicity or the like in the data transfer provided by communication with a customer via a digital circuit.
In the on demand print technique, digital copying machines have been used for printing a catalog an instruction manual of a product, documents distributed in an office or the like, as their image quality has been improved to the level comparable to printed materials. An image forming system which uses a copying machine accommodating on demand printing has been composed of a plurality of large capacity recording medium feeding apparatuses sequentially connected to address various types of materials.
Further, there is an image forming system for performing various post-processes on a recording medium (hereinafter, merely called as a “sheet”.) outputted from an image forming apparatus. For example, a Z-folding process (an A3 sized sheet is Z-folded into an A4 sized sheet.) or an inserter process to insert a sheet between sheets is known. A large image forming system that performs post-processing steps such as a staple process for bundling sheets, a perforation process for perforating a sheet or a book binding process in a series of job operation is proposed.
FIG. 11 is a diagram for exemplifying an image forming system in which an image forming apparatus 1100 and a plurality of sheet feeding apparatuses (1110a-d), which are sheet processing devices for supplying sheets to the image forming apparatus 1100, are connected (sequentially connected). In such an image forming system as in FIG. 11, a conveyer channel from each sheet feeding apparatus to the image forming apparatus generally has a shared configuration. In such a configuration, if image forming is performed on a pamphlet consisting of a plurality of pages by using a plurality of kinds of sheets, the sheet feeding apparatus is also changed as the sheets are changed. For example, there may be a case where a sheet feeding apparatus nearest to the image forming apparatus (in FIG. 11, 1110a) is changed to a sheet feeding apparatus remotest from the image forming apparatus (in FIG. 11, 1110d).
In this case, a conveyer channel between the image forming apparatus 1100 and the sheet feeding apparatus 1110d is longer. Accordingly, with a sheet feeding process is performed at a usual sheet feeding timing, a conveying interval (distance between sheets) is longer than the case where sheets are fed from the sheet feeding apparatus 1110a, which causes a problem of lowering productivity of the system.
As a technique to solve the problem, a technique to change a feed timing or the like in consideration of the number, connections and arrangement of sheet feeding apparatuses to be connected to the image forming apparatus 1100 is proposed. As a technique of increasing the speed of conveying sheets on a conveyer channel to shorten the distance between the sheets, or as a technique of advance feeding such that feeding starts first from the sheet feeding apparatus remotest from the image forming apparatus without regard to the page order, a technique of improving productivity by keeping the interval between sheets the same.
When the advance feeding is performed, sheets are fed from each sheet feeding apparatus without regard to the pager order; many sheets are conveyed on a conveyer channel between a sheet feeing apparatus and an image forming apparatus.
If sheets are supplied by advance feeding without regard to the page order and a print process in response to an input from a user temporally stops, the page order deviates from the correct page order, for example an ascending order or a descending order, resulting in an erroneous page order. In such a case, sheets ejected after the last sheet of those ejected in the correct page order need to be removed and a feeding process needs to start at a sheet corresponding to the next page when a print process resumes.
In Japanese Patent Publication No. 8-27559, a technique about temporal suspension and resuming in a double-sided printing apparatus is disclosed for stopping and resuming of a print process. As Japanese Patent Publication No. 8-27559 is for keeping a second image forming operation (sheet feeding) for a sheet on which a first image forming operation has started so as not to accumulate sheets in the apparatus to stop the apparatus when a double-sided image forming is temporally suspended.
The configuration of Japanese Patent Publication No. 8-27559, however, is not for suspension and resuming due to advance feeding, and does not disclose anything about sheet feeding for image forming of a new first side.